The barium titanate takes a perovskite-type structure where barium and Ti occupy the A site and the B site, respectively, and titanium is slightly deviated from the center. By virtue of this structure, the barium titanate expresses ferroelectricity and in turn excellent electric properties such as dielectricity, piezoelectiricity and pyroelectricity and therefore, is being widely used as various electronic materials.
Also, the barium calcium titanate is widely used as a barium titanate-based electronic material resulting from replacing a part of barium by calcium and in particular, the barium calcium titanate is attracting attention as a ceramic capacitor material in which the temperature dependency and reliability of dielectric property of the barium titanate are improved.
Examples of the usage thereof include, by making use of high dielectricity of this material, various capacitor materials (including multilayer ceramic capacitor), dielectric filter, dielectric antenna, dielectric resonator, dielectric duplexer, capacitor and phase shifter, and also include stacked piezoelectric actuator utilizing piezoelectricity. As for the method of using the barium titanate in an electronic material, there is a method of mixing a barium titanate powder with a solvent to form a slurry or a paste, and shaping it into a thin film or a porcelain by molding, sintering, sheeting or the like.
In recent years, it is demanded to develop a barium calcium titanate having a small particle diameter, a narrow particle size distribution, excellent dispersibility, high crystallinity and high purity and being capable of coping with down-sized, lightweight and high-performance electronic parts.
For example, in order to obtain a downsized, lightweight and high-capacitance multilayer ceramic capacitor, the particle diameter of the barium calcium titanate must be made small, thereby decreasing the distance between layers stacked and increasing the number of layers stacked. In general, when the particle diameter of the barium calcium titanate becomes small, the c/a ratio decreases and the crystal comes close to a cubic crystal, but if the c/a ratio decreases, this leads to decrease in the ferroelectricity and on use as a dielectric material, decrease in the electrostatic capacitance. To solve this problem, a barium calcium titanate exhibiting a high c/a ratio despite a small particle is demanded. In order to exhibit a high c/a ratio, the barium calcium titanate must be highly crystalline.
Also, if the barium calcium titanate powder undergoes aggregation at the time of mixing the powder with a solvent to form a slurry or a paste, the sintering density decreases and in turn the electric properties such as breakdown voltage and migration deteriorate. Furthermore, impurities contained in the barium calcium titanate have adverse effect on the electric properties.
Accordingly, in order to cope with downsized, lightweight and high-performance electronic parts, development of a barium calcium titanate having a small particle diameter, a narrow particle size distribution, excellent dispersibility and high purity is necessary.
Conventionally, the barium titanate has been produced by the following process.
As for the production process of a high-purity high-crystalline particulate barium titanate, a flux process is known. However, in this process, not only the production cost is very high but also the particle formation can be effected only by grinding and therefore, the particles have a broad particle size distribution and poor dispersibility. Accordingly, this process is unsuitable for the electronic materials using a particle material.
As for the process of producing a barium titanate for electronic materials, there is generally known, for example, a solid-phase process of mixing powders of an oxide and a carbonate used as raw materials in a ball mill or the like and reacting the mixture at a high temperature of about 800° C. or more to produce a barium titanate; an oxalate process of preparing an oxalic acid composite salt and thermally decomposing the composite salt to obtain a titanium-containing composite oxide particle; a hydrothermal synthesis process of reacting raw materials in an aqueous solvent at high temperature and high pressure to obtain a precursor; and an alkoxide process of using a metal alkoxide as a raw material and hydrolyzing it to obtain a precursor.
Also, with respect to the production process of a barium titanate, for example, a process of reacting a hydrolysate of a titanium compound with a water-soluble barium salt in a strong alkali (Patent Document 1), and a process of reacting a titanium oxide sol with a barium compound in a strong alkali aqueous solution (Patent Document 2) are generally known, and studies are being aggressively made to improve these synthesis methods.
The solid-phase process has a problem that despite low production cost, the produced particulate barium titanate has a large particle diameter. If this particle is ground, the particle diameter may be decreased but the particle size distribution is broadened and the shaping density may not be enhanced. Furthermore, distortion may be generated in the crystal structure due to the effect of grinding and a barium titanate powder suitable for down-sized high-performance electronic parts may not be obtained.
In the oxalate process, a smaller particle than that produced by the solid-phase process is obtained, but a carbonate group derived from the oxalic acid remains as an impurity. Furthermore, a large amount of a hydroxyl group attributable to water taken into the inside remains and this deteriorates the electric properties. In this way, the oxalate process is disadvantageous in that a barium titanate powder with excellent electric properties cannot be obtained.
The hydrothermal synthesis process has a problem that although a fine particulate barium titanate is obtained, a hydroxyl group attributable to water taken into the inside remains to cause many defects and a particulate barium titanate with excellent electric properties can be hardly obtained. Also, this process is performed under high-temperature high-pressure conditions and therefore, exclusive equipment is necessary and the cost rises.
In the alkoxide process, a barium titanate finer than that produced by the hydrothermal synthesis process is obtained, but a hydroxyl group attributable to water taken into the inside remains to cause many defects and a titanium-containing composite oxide with excellent electric properties can be hardly obtained.
Patent Document 1: JP-B-3-39014 (the term “JP-B” as used herein means an “examined Japanese patent publication”)
Patent Document 2: International Patent Publication WO00/35811, pamphlet
Patent Document 3: International Patent Publication WO03/004416, pamphlet
Patent Document 4: JP-A-2002-60219 (the term “JP-A” as used herein ++-means an “unexamined published Japanese patent application”)
Patent Document 5: JP-A-2003-48774